This invention relates to an apparatus for forming a thin film.
Conventionally, there are known thin film forming methods such as CVD or PVC and proposed various apparatuses for forming a thin film using these methods.
In an attempt to form at low temperatures a thin film which has excellent adhesiveness, Applicant once proposed an apparatus for forming a thin film which included a source of evaporation, a counter electrode holding a substrate which opposes the source of evaporation, a filament disposed between the source of evaporation and the substrate for generating thermions, and a grid disposed between the filament and the counter electrode and impressed with positive potential relative to the counter electrode (Published Unexamined Japanese Patent Application 89763/1984).
According to this apparatus, substances evaporated from the source of evaporation are ionized by thermions from the filament, the ionized substances are accelerated by an electric field established between the grid and the counter electrode and impinged on the substrate to thereby form a thin film. If the potential of the grid relative to the counter electrode is too high, ions may be excessively accelerated to thereby damage the thin film. Therefore, the potential of the grid is desired to be lowered as much as possible, which, however, cannot strongly accelerate thermions from the filaments, the density of thermions having energy suitable for that ionization is reduced, the rate of ionization of evaporated substances is reduced, so that problems would possibly arise in the adhesiveness of a thin film to be formed to the substrate and in the crystalline property of the thin film. Adjustment of the grid potential for forming a good thin film is troublesome in this apparatus.